I don't know if any of this advice will apply to you, but I get to mark a few exams (even less fun than doing them - you only have to answer each question once!), and have seen the same screw ups time and time again. I now give a lecture to the students telling them exactly how they're going to screw up, and they still make the same dumb, basic mistakes.
Rule number 1: ANSWER THE QUESTIONS.
If you don't answer enough parts of questions to add up to the pass mark, then you don't pass. Each year I see papers where the student has revised part of the sylabus, answerd 1 question decently, but been totally unable to answer any other questions. Make sure you're going to be able to answer ALL the questions
Rule number 2: ANSWER THE QUESTIONS
(very similar to rule 1) The questions are often written to give you hints as to how they want them answered - for example sugguestion a technique you should apply to the problem. DO IT THE WAY YOUR TOLD. After you've answered the question, re-read it and make sure you've hit all the sub-points, and hints - there are marks for all of those.
Rule 3: If I can't read it I can't mark it
Paper isn't in short supply. If you screw up, rewrite it, rather than trying to fix it. Leave space, start a new page if there's not room. It's not fun trying to mark equations crammed into the margin because you ran out of space on the page. If necessary rewrite the whole answer in a clearer format, if your first go isn't clear. Strictly speaking there aren't marks for presentation, but when you have a huge pile of papers to mark, an unreadable answr makes a grumpy examiner.
Rule 4:Nail all the easy marks
There ARE easy marks... If a question has 10 marks, you'll get a couple of even attempting it. A half decent answer will get you 6. A GOOD answer will get you 8 or 9. That last mark to get 10/10 is almost impossible. Even on a maths exam there are makrs for technique and style - the right answer might only get you 2/10 if your working is poor. It therefore pays to make sure you have a good go at all the parts of the question (and bear that in mind when choosing your questions), rather than picking an option question where you can nail one part, but will struggle on the other parts - answer all that parts of the question.
Rule 5: Use your time well
Take time to read over the paper, and choose your questions. Work out how long you have for each question, spend that time on it. You probably don't have any rush. When you're finished re-read your answers, and improve them. I'm amazed every year that students would leave and exam early - do they really believe their answers are so perfect they can't be improved?
These may not all apply in your case, but I've marked a fair few papers over the years - most of the fails have made several of these mistakes.
Ian